What is the proper use of a high-occupancy vehicle (HOV) or carpool lane?

A high-occupancy vehicle (HOV) or carpool lane is reserved for vehicles carrying at least a posted minimum number of occupants (such as two or more people), and sometimes for other specifically permitted vehicles like buses, motorcycles, or qualifying clean-air vehicles, during the hours shown on the signs. HOV lanes encourage carpooling to reduce congestion. Entering or leaving the lane is typically allowed only at designated points, and crossing solid lines into the lane improperly is a violation. Using the HOV lane without meeting the occupancy requirement during restricted hours can bring a fine. Understanding HOV-lane rules is part of lane-use content.